The invention relates to aluminum-lithium alloys and more particularly, it relates to recycling of aluminum-lithium alloy scrap.
The use of aluminum-lithium alloys in aerospace applications is entering an accelerating growth phase. The lower density and increased stiffness associated with aluminum-lithium alloys can result in significant weight savings. For example, alloy 2090 provides 8% lower density and a 10% higher modulus than the conventional aluminum alloys. These weight savings enable the airframe designer to provide an aircraft with either increased range or payload, or a faster, more maneuverable aircraft. Aluminum-lithium alloys are being evaluated by all the major airframe manufacturers, and their application to structural components is expected to grow significantly.
The amount of aluminum-lithium scrap that will be generated by airframers depends on variables such as the buy-to-fly ratio and the degree of segregation. Normal buy-to-fly ratios range from 2:1 to 4:1 depending on the type of fabrication. Even at poor buy-to-fly ratios, and some approximate segregation, aluminum-lithium alloys will become a large problem in the aluminum scrap market.
Aluminum-lithium scrap causes remelting concerns if handled in the regular scrap loop. Some of the problems are: metallurgical property changes in recovered alloys; adverse effects on refractories; adverse effects on metal cleanliness; higher melt losses to dross; safety and industrial hygiene problems; environmental issues; and higher cost of recycling/recovery.
Lithium is considered an impurity in commercial nonlithium aluminum alloys. As an impurity, lithium falls into the "others each" category of the operating limits, generally 0.05%, for 2XXX to 6XXX series alloys. Thus, the maximum theoretical level of lithium that could be tolerated and still be acceptable is 0.05%; however, in practice, the lithium level could not attain this level due to the 0.15% limit imposed for "others total".
In commercial aluminum alloys, it is known that concentrations of lithium of less than 0.0005% (5 ppm) can promote discoloration in foil under humid conditions and increase the oxidation rate of molten aluminum as well as alter surface characteristics of wrought products.
It has been suggested that lithium impurity levels in the 0.005-0.1% range have an overall adverse effect on the properties such as strength and ductility of aluminum alloy wrought products. To what degree the relative amounts of lithium in this range affect properties is presently unknown.
Studies performed on standard alloys at the 0.1% lithium impurity level attribute strength and ductility loss to increased gas content (H.sub.2) and formation of lithium bearing phases (e.g., LiH) at the grain boundaries.
The present method for recycling lithium-containing aluminum scrap can recover 95% of the lithium from the scrap. Lithium in the recovered metal can be less than 10 ppm or 0.001 wt. %.